1. Field of the Invention
The present invention relates to a layer 2/layer 3 switch, transmission device, or other network device provided with an Ethernet (registered trademark, same below) interface (Ethernet device), more particularly relates to a network device designed to improve the operation of an auto negotiation function supported by that device used in a layer 2/layer 3 network over which carriers are primarily launching new services. Note that the auto negotiation function is a function for automatic selection of a communication mode among network devices and is defined by IEEE 802.3.
2. Description of the Related Art
Along with the faster speeds and broader bands of data communications in recent years, demand for services by inexpensive Ethernet interfaces able to accommodate user interfaces as well has been soaring.
At the present time, connection of Ethernet interfaces used by carriers requires use of the auto negotiation function defined by IEEE 802.3. This auto negotiation function is a function for automatic selection of the communication mode and is for realizing so-called “plug & play” connection not requiring any manual setting, that is, enabling immediate communication after being plug-in. With this function, two network devices exchange information on the communication modes such as the line speeds supported and can automatically select the optimal communication mode for the two. Examples of the communication modes are given next.                i) Case of 10/100 Base-TX                    Line speed (10 Mbps or 100 Mbps)            Duplex (full duplex or half duplex)            Flow (flow control on or flow control off)                        ii) Case of 1000 Base-X                    Flow (flow control on or flow control off)            Line speed (1000 Mbps fixed)            Duplex (full duplex fixed)                        
As explained above, the auto negotiation function is an effective function for realizing plug & play connection between network devices, but further improvement of its operation is demand to further improve its effectiveness.
Note that as known art relating to the present invention, there is Japanese Unexamined Patent Publication (Kokai) No. 10-271143. While this known art, however, is the same as the present invention from the viewpoint of relating to a “negotiation function”, it specifically relates to a negotiation function for TCP connection, so in fact differs from the present invention. The present invention relates to auto negotiation for selecting a communication mode at a physical layer.
The above auto negotiation function supported by the network devices (Ethernet devices) has the problem that the interconnectivity is not perfect, that is, there is a problem in connectivity between devices. This is due to the vagueness of the provisions of IEEE 802.3 and has resulted in the various manufacturers supplying physical layer chips (PHY chips) with different specifications and consequently the incompleteness of connectivity when setting (turning on) auto negotiation. Note that this problem will be explained in more detail later while referring to the drawings.